In recent years, in a wireless communication system using long term evolution (LTE) for example, taking measures to accommodate more traffic and improve communication quality has been desired because the traffic is steadily increasing. Thus, it has been studied to use the LTE technology in an unlicensed band (hereinafter referred to as “U-band”) that is used for a wireless local area network (LAN) and the like for example and needs no license.
Specifically, there is a technology referred to as licensed assisted access (LAA) that uses the LTE technology in the U-band. The LAA is a technology to transmit and receive data in the U-band by subsidiarily using a licensed band (hereinafter referred to as “L-band”) that is used in a wireless communication system such as a mobile telephone network for example and needs a license.
In the case that the LAA is employed, it has been considered, for example, that control data and the like is transmitted and received in the L-band and user data of best-effort type and the like is transmitted and received in the U-band. Then, as a control channel of downlink that transmits the control data to a terminal device from a base station apparatus, an enhanced-physical downlink control channel (EPDCCH) that has been standardized by 3GPP and others is available, for example. The EPDCCH is different from a physical-downlink control channel (PDCCH) that extends over the entire frequency band of the downlink, and as with a physical-downlink shared channel (PDSCH), uses a part of the frequency band as the control channel.
Incidentally, because the L-band is a frequency band that needs a license, a telecommunications carrier who has acquired the license occupies a specific frequency band that belongs to the L-band and the interference with the communications in other wireless communication systems does not occur. Meanwhile, because the U-band needs no license and is also used by other wireless communication systems such as a wireless LAN, the LAA that a different telecommunications carrier operates, and others for example, when the wireless communication using the U-band is performed, the interference with the communications in the other wireless communication systems can occur. Thus, when the LAA is employed, it is preferable that a scheme that prevents the interference with other communications be implemented. That is, like carrier sense for example, it is conceivable that, when a device performs wireless communication, the listen before talk (LBT) that checks whether the wireless communication is being performed by the other device is implemented. When implementing the LBT, the device that performs transmission detects received energy in the frequency band used for the transmission and determines whether the transmission by the other device is being executed. If the transmission by the other device is not being executed, then this frequency band is used and the transmission of data is executed. Accordingly, this makes it possible to prevent the interference caused by the collision of communications between the devices that transmit data in the identical frequency band.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2006-186992
Patent Literature 2: Japanese National Publication of International Patent Application No. 2014-500685
Patent Literature 3: Japanese Laid-open Patent Publication No. 2002-124916
Patent Literature 4: Japanese National Publication of International Patent Application No. 2008-508774
Non-Patent Literature 1: 3GPP TS 36.211 V12.3.0 September 2014
As described above, when performing communication using the U-band, it is preferable that the LBT be executed to prevent the interference with the communications in the other wireless communication systems. Then, when the LBT is executed, depending on the execution result, it is determined whether to actually transmit data via the U-band. That is, as a result of the LBT, when it is checked that the other device is not transmitting data, the transmission of data via the U-band is executed, and when the other device is transmitting data, the transmission of data via the U-band is deferred. Accordingly, until the execution result of the LBT is obtained, it is not determined whether the data is actually transmitted via the U-band.
Because of this, there is a problem in that, concerning the mapping of downlink control information (DCI) indicative of the allocation of data of downlink to a control channel for example, it is difficult for the base station apparatus to keep the DCI about the L-band and the U-band mapped to the control channel in advance.
That is, although the allocation of the data of downlink is determined in advance on the L-band by scheduling, it is not determined, on the U-band, whether the data is actually allocated to the downlink until the execution result of LBT is obtained. Consequently, when the base station apparatus keeps the DCI about the L-band and the U-band mapped to the control channel such as an EPDCCH in advance for example, depending on the execution result of the LBT concerning the U-band, the data is not allocated as per the DCI in some cases.
Specifically, when the DCI about the L-band and the U-band is mapped to the EPDCCH in advance for example, if the transmission of data via the U-band is deferred due to the execution result of LBT, the allocation of data for the U-band is no longer present. Accordingly, the DCI mapped in advance is no longer appropriate, and the DCI about only the L-band is to be mapped to the EPDCCH again.
As in the foregoing, because the presence of the allocation of data for the U-band is not determined until the execution result of LBT is obtained, it is difficult for the base station apparatus to keep the DCI mapped for the control channel. Thus, it is also conceivable to map the DCI about the L-band and the U-band to the control channel, after having obtained the execution result of LBT. In this case, however, in order to complete the mapping of the DCI about the L-band and the U-band in a short time before the sub-frame at which the data is actually transmitted, the need to speed up the mapping processing of the DCI arises. As a result, the hardware of high-performance or the like is desired, and that leads to an increase in cost.